Extruded consumable anodes with anodized core-cladding interface

ABSTRACT

AN EXTRUSION PROCESS FOR FORMING CONSUMABLE ANODES AND THE PRODUCT PRODUCED THEREIN. THE PROCESS COMPRISES THE CO-EXTRUSION OF A WIRE CORE HAVING A NON-FERROUS LIGHT METAL SURFACE WITH AN OVERCOATING OF A DISSIMILAR LIGHT METAL. A SYNTHETICALLY PRODUCED NON-FERROUS METAL OXIDE LAYER IS INTERPOSED BETWEEN THE DISSIMILAR LIGHT METAL SURFACES TO REDUCE THE INCIDENCE OF CORE WIRE BREAKAGE DURING EXTRUSION OPERATIONS. IT IS BELIEVED THAT ALLOYING OF THE LIGHT METAL CORE SURFACE WITH THE DISIMILAR EXTRUDED OVERCOATING IS INIBITED BY OXIDE LAYER, THEREBY SUBSTANTIALLY ELIMINATING ADHERENCE OF A LOW MELTING, NON-FERROUS ALLOY TO THE EXTRUSION DIE. BREAKAGE OF THE CORE WIRE DURING EXTRUSION IS THEREBY MINUMIZED.

United States Patent 3,723,282 EXTRUDED CONSUMABLE AN ODES WITH ANODIZED CORE-CLADDING INTERFACE John F. Pashak, Linwood, Mich., assignor to The Dow Chemical Company, Midland, Mich. No Drawing. Filed Nov. 23, 1970, Ser. No. 92,305 Int. Cl. C23f 13/00 U.S. Cl. 204-197 Claims ABSTRACT OF THE DISCLOSURE An extrusion process for forming consumable anodes and the product produced therein. The process comprises the co-extrusion of a wire core having a non-ferrous light metal surface with an overcoating of a dissimilar light metal. A synthetically produced non-ferrous metal oxide layer is interposed between the dissimilar light metal surfaces to reduce the incidence of core wire breakage during extrusion operations. It is believed that alloying of the light metal core surface with the dissimilar extruded overcoating is inhibited by the oxide layer, thereby substantially eliminating adherence of a low melting, non-ferrous alloy to the extrusion die. Breakage of the core wire during extrusion is thereby minimized.

BACKGROUND OF THE INVENTION This invention relates to extrusion and more specifically to the extrusion of consumable anodes and the products produced thereby.

Anodes have been produced by various methods including casting, extruding, and fabrication of the individual components. The completed anode product is commonly used to minimize corrosion by cathodic protection of water heaters, underground metallic pipes, or ships hulls. In operation, the consumable anode is usually positioned so as to be in electrical contact with the metal to be protected. Protection of the metal is effected by prefferential corrosion attack of the anode.

Previously, metals such as aluminum, magnesium, zinc, alloys thereof and combinations of these metals have been advantageously used in the preparation of consumable anodes. Additionally, light metal anodes having ferrous ar aluminized ferrous cores are known. A method for producing an extruded magnesium anode with an aluminum coated steel core wire is described in U.S. Pat. 2,841,546. The useful life of an aluminized steel cored magnesium anode is often superior to the simple ferrous cored anode.

One of the major difficulties encountered in producing bimetallic aluminum-magnesium overcoated black iron or steel Wire anodes is breakage of the wire during extrusion. The wire breakage has been so extreme and costly that processing using the aluminized ferrous wire with a nonferrous cladding, or a bimetallic aluminum-magnesium combination has not been generally accepted.

It is an object of this invention to provide a method for co-extrusion of dissimilar non-ferrous metals without excessive breakage of the core wire during extrusion.

Another object is to provide a method for extruding ferrous wire having a light metal coated surface and a dissimilar non-ferrous metal overcladding without excessive breakage during extrusion.

Other objects and advantages will become apparent during the course of the following description.

SUMMARY OF THE INVENTION The aforementioned difficulties have been surmounted and objects realized by the instant invention which comprises passing a continuous metal core, having a light metal surface, through a die of larger cross-section than the metal core and simultaneously extruding a light metal with the metal core, whereby an overcoating of light metal is applied to the continuous metal core; the light metal surface of the metal core having been anodized to produce a synthetic oxide layer prior to its passing through the extrusion die. The light metals used in this method can be dissimilar. Employable dissimilar light metals are alloys of either aluminum or magnesium containing at least 50% of the base metal.

Artifically forming an oxide coating of a light metal on the core surface or undercoating in which the inner core is positioned, was found to satisfactorily inhibit alloying of aluminum and magnesium during subsequent extrusion. The synthetically oxidized protective film, therefore, permits contact of the dissimilar non-ferrous metal surfaces at extrusion temperatures and pressures without the formation of a suflicient quantity of an aluminummagnesium alloy, which melts at a lower temperature than either of the alloy constituents, to result in enough build up in the extrusion die orifice to cause core wire breakage.

An extrusion is produced which comprises a metal core having a light metal surface and an overcoating of a light metal. At least one of the interfacing surfaces between the light metal surface of the core and the inner surface of the cladding has a coating of a synthetically produced oxide of the underlying light metal. The light metal in the consumable electrode is usually an aluminum alloy or manganese alloy.

It is postulated that at least part of the success of this product results from the oxide coating being discontinuous. While the synthetically produced oxide coating can be continuous on the wire prior to extrusion the forces incurred during extrusion cause random cracking or fracturing of the oxide coating. An aluminum-magnesium intermetallic composition having a low melting point is generally formed at these randomized fractures to provide adequate electrical contact between the overcoating and the undercoating to create an effective anode.

PREFERRED EMBODIMENT In the process of the current invention it is preferred the core material be a ferrous material coated with an aluminum alloy. The aluminum coating on the ferrous core may be achieved by a means such as spraying, dipping, or casting the aluminum around the wire. Examples of suitable materials for ferrous base wires are steel and black iron. Commercially purchased aluminized black iron wire can also be used successfully in this process.

The specific method of forming oxide on the nonferrous inner layer adhering to the exterior of the ferrous core wire or to the exterior surface of the non-ferrous base inner core wire is not critical. Examples of suitable oxide application methods are anodizing, and heating in an oxygen containing atmosphere. It is of importance though that the oxide layer be at least 0.0002 inch thick to adequately protect against alloying of the magnesium and aluminum, while being extruded at temperatures and pressures common in the extrusion process. Preferably the oxide layer is present to a depth of about 0.0004 inch to about 0.001 inch. Oxide coatings of greater thickness than 0.001 inch are employable in this invention, however, the additional depth is believed to be superfluous. A sufiicient coating of oxide must be present on the light metal surface to hinder the accumulation of enough intermetallic aluminum-magnesium composition in the die orifice to cause wire breakage during extrusion.

To achieve the greatest benefit from the instant invention it is preferred that a magnesium base alloy overcladding surround the generally axially aligned oxidized aluminized ferrous wire. This combination will provide improved strength of the completed anode and retain 3 the beneficial corrosion protection properties of magnesium.

Coating the generally axially aligned iron base core with a magnesium alloy, anodizing the magnesium layer, and co-extruding the magnesium coated iron base wire with an outer layer of an aluminum alloy will also produce acceptable results. A magnesium undercoating can be achieved by casting a magnesium alloy around a zinc galvanized iron base wire.

Example 1 An aluminum base alloy, registered with The Aluminum Association as 5356, having a nominal composition of 5.0% magnesium, 0.12% manganese, 0.12% chromium, and 0.12% titanium was employed in accord with this invention. A 5356 alloy wire inch diameter by ten feet long was anodized in sulfuric acid to produce an oxide coating 0.001 inch thick on the surface of said wire. After anodizing, the wire was fed into an orifice in a three port anode extrusion die in a 500 ton laboratory extrusion press to form an anode core. A magnesium alloy containing at least 99.5% magnesium was extruded around the anodized 5356 alloy aluminum wire to produce a one inch diameter anode.

The extrusion die and extrusion billet containers were preheated to a temperature of 900 F. The magnesium billets were preheated to a temperature of 950 F. prior to extruding. Continuous extrusions having a magnesium overcoating and 5356 alloy aluminum core wire were produced without wire breakage. The die orifice was examined and the previously common aluminum-magnesium intermetallic compound was not apparent.

Examples 26 Aluminum 5356 alloy wire was positioned within an orifice in a three port extrusion die in a 500 ton extrusion press and simultaneously extruded with a magnesium alloy having an ASTM designation of AZ31B and a nominal composition of 3% aluminum, 1% zinc, and 0.4% manganese. Prior to extrusion the 5356 wire was anodized in sulfuric acid by standard procedures to cause an oxide surface layer to form. Depths of the oxide layers utilized herein are shown in the table. AZ31B ingot was preheated to a temperature of 850 F. before being passed through a die orifice heated to 800 "F. at a speed of five feet per minute.

The extruded product was a one inch diameter composite having an anodized 5356 alloy core and a cladding of AZ31B magnesium alloy. An anode product was extruded employing aluminum core wire, which had been anodized to form aluminum oxide on the surface to the hereinafter listed depths, without wire breakage.

EXTRUDABILIT'Y OF ANODIZED 5356 ALUMINUM CORE AND MAGNESIUM AZ31B CLADDING H1804 anodized core What is claimed is:

1. A consumable anode comprising a metal core with a light metal surface and an overcoating of a light metal, at least one of the interfacing surfaces between the surface of the core and the inner surface of the overcoating having at least a 0.0002 inch thick oxide coating of the underlying light metal, each of said light metals being selected from the group consisting of aluminum alloys and magnesium alloys.

2. The consumable anode of claim 1 wherein said metal core is an aluminum alloy and the overcoating is a magnesium alloy.

3. The consumable anode of claim 2 wherein said oxide is aluminum oxide.

4. The consumable anode of claim 1 wherein said oxide is from about 0.0004 to about 0.001 inch in depth.

5. A consumable anode comprising a ferrous base metal core with a first coating layer of a light metal and a second overcoating layer of a light metal at least one of the interfacing surfaces between the first and second light metal layers being coated with at least a 0.0002 inch of an oxide of the underlying light metal, each of said light metals being selected from the group consisting of aluminum alloys and magnesium alloys.

6. The consumable anode of claim 5 wherein the first coating layer is an aluminum alloy.

7. The consumable anode of claim 6 wherein the overcoating is at least about percent magnesium.

8. The consumable anode of claim 5 wherein the overcoating layer is a magnesium alloy.

9. The consumable anode of claim 5 wherein the oxide is from about 0.0004 to about 0.001 inch in depth.

10. The consumable anode of claim 9 wherein the first coating layer is an aluminum alloy, the overcoating layer is a magnesium alloy and the oxide is aluminum oxide.

References Cited UNITED STATES PATENTS 2,478,478 8/1949 Grebe 204-197 2,841,546 7/1958 Robinson 204-l97 2,895,893 7/1959 Robinson 204-197 3,312,536 4/1967 Broverman 29197 TA-HSUNG TUNG, Primary Examiner U.S. Cl. X.R. 204148 

